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Chemical Bonds: Ionic and Covalent Compounds

This chapter explores the concepts of ionic and covalent bonding in chemical compounds. It covers the octet rule, naming of compounds, Lewis dot structures, molecular geometry, and polarity.

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Chemical Bonds: Ionic and Covalent Compounds

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  1. Chapter 3 Chemical Bonds

  2. Note: order changed slightly • Will start with ionic bonding and ionic compounds, then move to covalent bonding and molecular compounds, where Lewis dot structures are used • Then will do molecular geometry and VSEPR theory and discuss polarity

  3. Key topics for this chapter • Octet rule and how it relates to quantum model of the atom • Differentiating between ionic and covalent bonds • Naming of ionic and molecular compounds • Use of Lewis dot structures and VSEPR theory to determine bonding and geometry of molecular compounds • Bond polarity • Molecule polarity

  4. Major themes • Octet rule: valence electrons make chemistry happen, core electrons play small role • Metals react with nonmetals to form ionic compounds • Nonmetals react with nonmetals to form molecular compounds • We learn the rules for writing names and formulas for ionic and molecular compounds • We learn about molecular shape and polarity

  5. Octet Rule

  6. Octet rule

  7. Electron Configurations of Cations and Anions Of Representative Elements Na [Ne]3s1 Na+ [Ne] Metal Atoms ___________ electrons so that cation has a noble-gas outer electron configuration. Ca [Ar]4s2 Ca2+ [Ar] Al [Ne]3s23p1 Al3+ [Ne] H 1s1 H- 1s2 or [He] Nonmetal Atoms ____________ electrons so that anion has a noble-gas outer electron configuration. F 1s22s22p5 F- 1s22s22p6 or [Ne] O 1s22s22p4 O2- 1s22s22p6 or [Ne] N 1s22s22p3 N3- 1s22s22p6 or [Ne] 8.2

  8. -1 -2 -3 +1 +2 +3 Cations and Anions Of Representative Elements 8.2

  9. Types of Chemical Bonds • According to the Lewis Model Ionic Bonding • An atom may lose or gain enough electrons to acquire the same electronic structure as its nearest noble gas neighbor (usually 8 valence e-) and become an ion • An ionic bond is the result of the force of attraction between a positive ion (cation) and a negative ion (anion) • Occurs between metals and nonmetals

  10. Ionic Bonds

  11. Formation of Ionic Bonds • Occur when electrons are transferred from a metal to a nonmetal

  12. Ionic Bond in NaCl

  13. Ionic Bond in CaCl2

  14. Formation of Ionic Bonds • Each atom attains a stable electronic structure • Number of protons/electrons in atoms no longer equal, charged particles result (ions) • Metals lose e-, form + ions (cations) • Nonmetals gain e-, form – ions (anions)

  15. A Closer Look at Salt

  16. Salt contains a large number of Na+ and Cl- ions

  17. Ions pack together tightly into crystalline solids • Strong attractive force between anions and cations holds crystal together

  18. Types of Chemical Bonds • According to the Lewis Model Covalent Bonding • An atom may share electrons with one or more other atoms to acquire a filled valence shell. • A covalent bond is the force of attraction between two atoms that share one or more pairs of electrons. • Occurs between nonmetals

  19. Covalent Bonds • A sharing of valence electrons between atoms • Usually occurs between nonmetallic elements • Nonmetals need to gain e- in order to attain a stable e- structure • Thus, when they combine, end up sharing valence electrons

  20. Covalent Bond Atom 1 Atom 2 • Attractive forces between nucleus of 1 atom and e- from other atom • Repulsive forces between nuclei, electron clouds • Attractive forces stronger, bond forms

  21. Covalent Bonds Bohr model of covalent bond • By Sharing e- • hydrogen obtains a completely filled 1st energy level • oxygen obtains a completely filled 2nd energy level (stable octet)

  22. Lewis, or electron, Dot Diagrams • Show the valence electrons • Consist of element symbol – represents nucleus and core electrons – and dots (valence e-)

  23. Elements and Covalent Bonds • Most nonmetallic elements do not exist in nature as individual atoms • e.g. Hydrogen exists as a diatomic molecule, H2 • By overlapping 1s orbitals, each H atom attains a stable e- structure

  24. Other Molecular Elements Diatomic Elements(Br2, I2, N2, Cl2, H2, O2, F2) • Hydrogen • All group 7A elements • Oxygen • Nitrogen BrINCIHOF!!! Memorize it

  25. General Rules for Bond Types Negative complex ions Nonmetal Nonmetal Covalent Bond Ionic Bond Metal

  26. What determines bond type? Generalization • metal + nonmetal = ionic bond • nonmetal + nonmetal = covalent bond Reality • Bonding type is on a continuum, from 100% ionic to 100% covalent 100% covalent 100% ionic

  27. Determining Predominant Bond type • Electronegativity is a measure of the tendency for atoms of an element to attract electrons in a chemical bond. • Originated with American chemist Linus Pauling (1901-1994), a 2x Nobel Prize winner from Stanford U - a relative scale

  28. Electronegativity (EN) Scale ranges from 0.7 to 4.0

  29. Electronegativity (EN) Trends in Periodic table

  30. Highly electronegative elements (N, O, F, Cl) are greedy for electrons • Predominant type of bonding (ionic/covalent) determined by differences in electronegativity (EN) between atoms in bond • EN > 1.9, predominantly ionic (metal, nonmetal) • EN 0.5 to 1.9, polar covalent • EN < 0.5, nonpolar covalent Nonmetal, nonmetal

  31. Determining Predominant Bond Type • Look at the electronegativity values of atoms involved in bond • Calculate electronegativity difference (EN) • Draw arrow in direction that e- cloud is pulled (more electronegative atom) • Also use lower case Greek symbol for delta, , along with + or - sign

  32. Example: Bond between H and Cl • Electronegativity values H= 2.1, Cl = 3.0 • EN = 3.0-2.1 = 0.9 = polar covalent bond • e- cloud pulled towards Cl atom = Polar Covalent Bond Cl H Cl H d+ d- e- rich electron poor region electron rich region 2.1 3.0 e- poor Direction e- cloud being pulled 9.5

  33. Result of differences in electro-negativity: • More electronegative element has greater share of e- cloud, it is electron rich • Less electronegative element is e- poor • Creation of bond dipoles: +and  -

  34. Example of Nonpolar Covalent Bonds • Diatomic elements e.g. H2 • Bonds between P and H = 2.1-2.1=0

  35. Bond Character Summary • Nonpolar covalent bond  electrons shared equally • Polar covalent bond  e- not shared equally, more electronegative atom has greater share of e- cloud • Ionic bond – e- transferred from one atom to other, creates a + and – ion

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